I've been restoring a PDP-11/05 recently and after replacing several faulty ICs I have it mostly working. I've run into a bit of a problem whilst running MAINDEC-11-D0NB (T14 TRAP TEST) though. The failing instruction sequence is: 7200:?? MOV #6340,R0 7204:?? MOV R0,(R0)+ 7206:?? CMP 6340,#6342 7214:?? BEQ 7220 7216:?? HALT This halts at 7216 with: ? R0 = 6342 ? 6340 = 6340 I tried this same set of instructions on a PDP-11/84 and also on Simh and the result is: ? R0 = 6342 ? 6340 = 6342 which is what the diagnostic seems to expect. I've carefully looked through the PDP-11/05 microprogram listing but I'm having difficulty seeing where this is going wrong. Here is a brief extract of the microprogram in the context of the MOV R0,(R0)+ instruction along with my interpretation of what I think is going on: LOC? NXT? * SOURCE MODE 0 (REGISTER), GET SOURCE DATA 201? 007? S0-1? B=R[S]; BUT BYTE 007? 001? S0-2? R[10]=B; BUT DESTINATION ????????? / IF IR<5:3> = 2 GOTO D2-1 ? B = R0 = 6340??? // B = source register ? R10 = B = 6340?? // Source data stored in sratch pad register R10 LOC? NXT? * DEST MODE 2 (AUTO-INC) GET DEST DATA, OP AND REPLACE 105? 331? D2-1? BA=R[D]; DATAIP; ALBYT 331? 341? D2-2? B=R[D]+1+BYTE.BAR 341? 200? D2-3? R[D]=B; BUT JSRMP; GOTO D1-2; CKOFF ????????? / IF INST NOT JMP OR JSR FALL THROUGH TO D1-2 ? BA = R0 = 6340??? // Bus address = destination register ? B = R0 + 2 = 6342 // Auto-increment and store in B ? R0 = B = 6342???? // Update destination register LOC? NXT? * DEST MODE 1 (REG,DEFERRED) GET DEST DATA, OP AND REPLACE 200? 210? D1-2? B=UNIBUS DATA; BUT BYTE 210? 143? D1-3? R[11]=B; BUT UNARY 163? 334? D1-4? B=R[10] OP B; BUT NOMOD 334? 065? D1-5? DATO; ALBYT; CKOFF 065? 305? D1-5? DRIVERS=B; GOTO S2-2 (BUT SERVICE) ? B = (6340) = 0??? // B = value at location pointed to by bus address ? R11 = B = 0?????? // R11 is only used for unary instructions ? B = R10 = 6340??? // B = source data stored previously in R10 ? (6340) = B = 6340 // B is written to the address pointed to by bus address Where have I gone wrong with this? I can't see from the above how the value at 6340 can possibly be 6342 Matt